Solid sorption refrigeration systems are driven by the adsorption and desorption of a refrigerant vapor (the adsorbate) by a solid substance (the adsorbent) which is usually highly porous. In contrast to conventional vapor-compression cooling systems which are driven by a mechanical compressor, no electrical energy is needed to drive the refrigeration cycle. The basic cycle involves an adsorption phase and a desorption phase. In the adsorption phase, the refrigerant vapor is adsorbed by the adsorbent substance resulting in release of heat. In the desorption phase, heat is applied to the adsorbent causing desorption of the refrigerant. The heat transferred during these processes is conveyed by a heat exchanger between the adsorbent and a heat transfer fluid (e.g. water or a water-glycol mixture) or an external environment. The adsorption and desorption processes occur in conjunction with evaporation and condensation of refrigerant in an evaporator/condenser. The adsorption of the gaseous refrigerant lowers the vapor pressure, promoting evaporation of the liquid refrigerant in the evaporator. During this evaporation, heat is extracted from an environment to be cooled, resulting in refrigeration. By supplying heat to the adsorbent via the heat exchanger, the adsorbed refrigerant is released into the vapor phase, thus regenerating the adsorbent material for the next adsorption cycle. The now gaseous adsorbate passes to a condenser where heat rejection to the environment takes place. As in conventional vapor-compression cooling, the liquid refrigerant is passed through a control device (e.g. an expansion valve) back into the evaporator, and the cycle can then be repeated.
The refrigeration cycle is driven entirely by heat input during the desorption phase without further input of mechanical work. Solid sorption cooling may therefore be preferred over conventional vapor-compression cooling where excess heat is readily available, e.g. from the sun (solar cooling) or as waste heat from other sources such as power dissipation in datacenters. Another advantage is that the usual refrigerants, such as water and methanol, used in solid sorption cooling are environmentally friendly and make a negligible contribution to global warming, unlike the fluorocarbon refrigerants typically used in vapor-compression cooling.
Despite the advantages, the cooling power of solid sorption cooling systems is very low compared to vapor compression cooling systems of the same size. The low specific cooling power is due, in part, to the poor heat transfer characteristics of the adsorbent material. In first-generation systems, the adsorber units used beds of loose adsorbent grains lying on the metal heat exchanger surface. Recent efforts to improve thermal conductivity of adsorbents include the use of monolithic adsorbents instead of granular adsorbent material and use of adsorbent materials with high thermal conductivity. Consolidated adsorbent materials have been formed of graphite, metallic foams, or adsorbent grains bound in a matrix such as resin. Composite adsorbent materials have also been produced from a mixture of adsorbent particles and particles with higher thermal conductivity. These consolidated materials generally suffer from poor permeability to the adsorbate vapor whereby flow of adsorbate within the structure is significantly hindered. Due to long diffusion paths to adsorption sites in granular or consolidated adsorbents, the vapor pressure near the inner adsorbent surface is slow to adapt to variations in the system pressure, thereby limiting the rate of cooling and regeneration of the adsorbent. More recently, heat transfer at the thermal interface between the heat exchanger and adsorbent material has been improved by coating the heat exchanger fins or tubes with thin-film adsorbent coatings formed by adsorbent material dispersed in a suitable matrix or by direct synthesis of the adsorbent, usually a type of zeolite, on the external heat exchanger surface.
US Patent Application Publication No. 2009/0217526A1 discloses an adsorption heat exchanger produced by applying an adhesive layer to a heat exchanger surface. After filling with granular adsorbent and application of heat, the adsorbent particles directly adjacent the heat exchanger are partially embedded in the adhesive. The remaining, unglued particles can then be removed to leave an adsorbent coating.
US Patent Application Publication No. 2006/0166819A1 discloses resin-bonded adsorbent materials for use as moulding compositions. U.S. Pat. No. 7,875,738 discloses consolidated compositions of particulate adsorbent material in a matrix of amorphous non-glassy ceramic material. Such a composition may be formed in a pipette tip by introducing a dispersion of binder polymers and particulate adsorbent in a solvent and then curing this dispersion to form the adsorbent mass.
Japanese Patent Application Publication No. 2000018767A discloses adhesion of adsorbent particles to a heat exchanger and to each other via a dispersion of thermosetting or thermoplastic adhesive powder to form randomly distributed point-like adhesive contacts.